Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface showing the new design; and,

FIG. 2 is a front view of the display screen or portion thereof with the graphical user interface showing the new design.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 and 2. The process or period in which one image transitions to another forms no part of the claimed design. The broken lines show aspects of the display screen or a portion thereof with a graphical user interface that are excluded from the claim and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 